1. Field of the Invention
This invention relates to pesticides and more particularly to biopesticides, including close derivatives and analogs of natural sources.
2. Background of the Art
Pesticides are used to increase the world's food supply and decrease disease-carrying pests. The large-scale use of pesticides both in agriculture and household pest control, however, has caused widespread concern regarding environmental impact, increased resistance, and both acute and chronic toxicity to non-target organisms, including man. Many pesticides, particularly the polychlorinated hydrocarbons such as DDT, have generated much controversy because they persist for years in the environment, and have proven harmful to fish and birds of prey. The concern over the buildup of nondegradable pesticides in the soil and in the food chain has prompted the Federal government to ban the production in the United States of many compounds such as DDT, and severely restrict the use of others.
Two widely used classes of pesticides which decompose more rapidly in the environment than polychlorinated hydrocarbons are the organophosphates and carbamates. Many of these compounds, however, are still considered highly toxic and their use is heavily regulated by the Federal government.
Pesticides can be applied through varying routes of exposure depending on the specific need, as well as the toxicity of the pesticidal agent itself. One commonly used vehicle for eliminating pests is to fumigate an infested area, plant or tree. Fumigation is defined as the use of a gas pesticide, or a volatile solid or liquid pesticide to control pests in buildings, ships, foods, plants, soil, and various stored products. Fumigants are also highly toxic and therefore subject to Federal regulation. Because of the need for high volatility in fumigant use, only about 20 chemicals are routinely used, including compounds containing chlorine, bromine and/or phosphorus. Fumigants may also be used in liquid form for injection into the ground to eliminate pests. There is growing concern, however, that persistence in the soil of certain fumigants is harmful to the environment. As a result, commonly used fumigants such as methyl bromide, dichlorvos, ethylene dibromide, chloropicrin, dibromochloropropane, carbon tetrachloride, phosphine, and 1,3-dichloropropene are heavily regulated and their use is severely restricted.
As part of the Federal government's regulation process, all pesticides must be registered. Since registration can be an extremely time-consuming and very costly process, there has been a marked decrease recently in the registration of new pesticides. This scarcity of new pest control agents has led to concerns that target pests will develop increased resistance to a very limited selection of products. Recently, the government started offering a "fast-track" registration process with the Environmental Protection Agency (EPA) for "biopesticides" which include both naturally-based pesticides and close derivatives or analogs. Although biopesticides offer a desirable alternative to the highly toxic pesticides, relatively few have been registered to date on the fast-track system.
One possible alternative is to use naturally-based pesticides. It is well-known that plants have built-in natural defenses against insects and other pests, which have evolved over time. Monoterpenoids, for example, are 10-carbon compounds composed of two isoprene units connected in a head-to-end manner. Monoterpenoids are found in the essential oils of many plants including mints, pine, cedar, citrus, eucalyptus and spices. Monoterpenoids are known to have the ability to aid plants in chemical defense against insects, bacteria, fungi, and even vertebrate herbivores. Many compounds in this class are used as flavors or fragrances in foods, cosmetics and pharmaceuticals. Further, there are several monoterpenoids that are commercially available for uses such as flea control on pets and carpets, control of insects on house plants, fumigation of parasitic mites in honey bee colonies, and insect repellency, such as with the citronella candle.
Another class of naturally-based pesticides are the glucosinolates. Within the family Cruciferae ("crucifer"), glucosinolates act as natural pesticides in many of the plants. Glucosinolates are a group of over 90 secondary metabolites that occur in only 11 families of dicotyledonous plants, mostly in the family Cruciferae. It has been shown that higher concentrations of glucosinolates correlate with less severe insect attacks. Of interest is the fact that although glucosinolates are reportedly toxic to livestock, they are clearly safe for humans. Specifically, within the crucifer family, glucosinolates are found in plants such as cabbages, radishes, turnips, mustard, collard greens, rape, broccoli, kale and crambe (Crambe abyssinica L.). It is the breakdown products of glucosinolates which are responsible for the pungent odor and biting taste of these plants.
Studies concerning effects of monoterpenoids and glucosinolates on insect pests have emphasized chemical ecological functions rather than acute toxicity. For example, glucosinolates have been found to play important roles in aiding certain insect species to identify their proper host plants. Experiments have shown that while too low a concentration of glucosinolates leads to ineffective larval attraction, too great a concentration may actually exert a repellent effect to the cabbage root fly. Others have demonstrated that several glucosinolates have a feeding stimulation effect on the diamond-back moth (Plutella maculipennis Curtis) and the larvae of Pieris brassicae.
The structure-activity relationship monoterpenoids and monoterpenoid derivatives have been studied as stated in the Abstract entitled "Insecticidal Properties of Monoterpenoids Against Some Insect Species," Sangkyun Lee, et al., Division of Agrochemical of the American Chemical Society, Fall 1993 and in "Toxicity and Neurotoxic Effects of Monoterpenoids in Insects and Earthworms," Joel R. Coats, et al., Chapter 20, American Chemical Society, 1991, which are hereby incorporated by reference. As biopesticides, monoterpenoids have been found to have mild insecticidal activity.
Glucosinolate-containing water extracts from crambe meal showing insect toxicity in laboratory tests are also known and discussed in "Insecticidal Toxicities of Glucosinolate-containing Extracts from Crambe Seeds," by R. Tsao, et al., 1995 which is hereby incorporated by reference. The acute toxicity of these extracts as pesticides, however, is significantly lower than conventional organochlorine or organophosphorus pesticides.
There is ongoing debate as to the risks and benefits posed by various natural versus synthetic pesticides. While it is true that many synthetic pesticides are quite toxic, other synthetic pesticides, such as allethrin and methoprene (which are actually biopesticides), are considered relatively safe. Furthermore, although many natural pesticides are relatively safe, there are clearly others, such as nicotine, which are recognized as being quite hazardous. It is now well-accepted that the biological activity of a chemical is a function of its structure rather than its origin, and the biological properties of a chemical depend on this structure as well as the manner in which the chemical is used. The "safety" of a pesticide, therefore, is dependent not only on the chemical make-up of the product, but also on the actual exposure to the chemical. In many cases, the perceived risk is not consistent with the actual risk.
What is needed, therefore, are new types of pesticides or biopesticides to replace those commercial products which are being banned, restricted or phased out, so that increasing resistance by target pests can be avoided and/or overcome. Further, the replacement pesticides need to be economical, highly toxic to target pests, and pose less actual risk to the environment and humans as compared to traditional pesticides.